Leakage-proof neutron diffractometer

ABSTRACT

A beam of neutrons passes through a hole disposed in a stationary shield and an opening disposed in a rotatable shield contained within the stationary shield and opposed to and diverging toward the hole. The beam impinges upon a crystal positioned within the rotatable shield. The crystal reflects the incident neutrons to form a beam of monochromatic neutrons which, in turn, passes through a radial hole disposed in the rotatable shield and a circumferentially wide slot disposed in the stationary shield and the beam of monochromatic neutrons is taken out from the slot.

United States Patent [72] Inventors Kazuo Yanagishita;

, Akira Iwakishi, Amagasaki, Japan [21] Appl. No. 716,969

[.22] Filed Mar. 28, 1968 [45] Patented Jan. 26, 1971 [73] Assignee Mitsubishi Denki Kabushiki Kaisha Tokyo, Japan [32] Priority Mar. 31, 1967 [54] LEAKAGE-PROOF NEUTRON DIFFRACTOMETER 2 Claims, 12 Drawing Figs.

[52], US. Cl, ,250/108, 250/84.5, 250/105 [51] Int. Cl G21k1/00 [50] Field ofSeai-ch 250/83.1, 845,108, 105

[56] References Cited UNITED STATES PATENTS 3,002,095 9/1961 Holcomb 250/108 3,268,730 8/1966 Van de Graaf 250/84 Primary Examiner-Archie R. Borchelt Attorneys-Robert E. Burns and Emmanuel J. Lobato PATENTED JAN26 I97! SHEET 1 [IF 2 FIG. lb

FIG. l0

(PRIOR ART) (PRIOR ART) m @A mm mb I .bm Q A F Fm 2 M w l1 1% f 5 0 V a 2 2 Q 8; n 1 w W\ LEAKAGE-PRQOF NEUTRON DIFFRACTOMETER The invention relates in general to a neutron diffractometer, and more particularly to improvements in shield devices for preventing neutrons from leaking out from neutron difiractometers.

There are known various types of shield devices incorporated into neutron diffractometers to prevent neutron leakage. Such shield devices all were not completely satisfactory in preventing neutron leakage from the associated neutron diffractometers or allowing their simple operation.

Accordingly, it is an object of the invention to provide an improved shield assembly for use in a neutron diffractometer to substantially completely prevent neutrons from leaking out from the diffractometer and suitable for use in operator-less continuous measurements for long times such as a measurement conducted with a beam of monochromatic neutrons varied in neutron wavelength.

Briefly, the invention accomplishes this object by providing a neutron diffractometer comprising a shield assembly including a stationary shield block with a narrow inlet passageway and a wide outlet passageway. A rotatable shield block is operatively coupled to the stationary shield block by a wide inlet passageway positioned opposite the stationary shield blocks narrow inlet passageway and a narrow outlet passageway in the shield is positioned opposite the stationary shield blocks wide outlet passageway. The rotatable shield blocks inlet and outlet passageways communicate with a central space .within the rotatable shield block in which is positioned an irradiated target crystal. A neutron beam is introduced into the ditfractometer through the stationary shield block's narrow inlet passageway to impinge on the irradiated target member positioned within the rotatable shield block. The incident neutrons are reflected by the irradiated target member through the rotatable shield blocks marrow outlet passageway and the stationary shield blocks wide outlet passageway to a device external to the diffractometer, for instance a neutron collimator.

In another embodiment a plurality of rotatable shield blocks may be vertically disposed in the hollow position of the stationary shield block. Each of the rotatable shield blocks is provided with a wide inlet passageway and a narrow outlet passageway positioned opposite and in alignment with one of a plurality of narrow inlet passageways and wide outlet passageways, respectively, vertically disposed in the stationary shield block.

The invention will become more readily apparent from the following detailed description in conjunction with the accompanying drawings in which:

FIG. 1a is a horizontal sectional view of a neutron diffractometer constructed in accordance with the principles of the prior art;

FIG. lb is a vertical sectional view taken along the line lb -lb of FIG. la;

FIGS. and b are views similar respectively to FIGS. la and b but illustrating another form of the conventional neutron diffractometers;

FIG. 3a is a horizontal sectional view of a neutron diffractometer constructed in accordance with the principles of the invention;

FIG. 3b is a vertical sectional view taken along the line lllb -Illb of FIG. 3a;

FIG. 3c is a vertical section view of another device taken along a section line similarly to FIG. 3b; and

FIGS. 40, b, c, d and e are side elevational sectional views of different forms of rotatable shield block constructed in accordance with the principles of the invention.

Throughout several FIGS. like reference numerals designate the corresponding or similar components.

Referring now to the drawings and FIGS. 1a and b in particular, there is illustrated one form of the conventional neutron diffractometers. An arrangement illustrated comprises a nuclear reactor including a reactor core 10 and a reactor wall 12 and an experimental hole plug 14 snugly fitted into a hole extending through the reactor wall 12. A hole 16 longitudinally extends through the hole plug 14 to provide a neutron passageway through which a beam of neutrons from the reactor passes. In order to prevent neutrons from escaping from the reactor, externally of the reactor wall 12, a stationary shield block 18 is attached to the reactor wall 12 and provided with a hole or a narrow inlet passageway 16a aligned with the neutron passageway 16 thereby extending the path over which the neutrons travel. The stationary shield block 18 is provided with a central cylindrical space 20 communicating with the inlet passageway 16a and the outlet passageway 22. The opening 22 is disposed in that part of the stationary shield block opposite the side contiguous with the reactor wall 12 and is in communication with the central space 20 and the space external to the shield block. A plurality of movable shield elements 24a, 12, c and d are detachably and replaceably disposed in the outlet passageway 22 with their internal end faces defining a part of the central space 20. One of the movable shield elements, in this case the end shield element 24d is separated from the other shield elements thereby defining a narrow radial outlet passageway 26 at the same level as the inlet passageway 16a for a purpose that will be later apparent.

In the central space 20 of the stationary shield block 18 is disposed a rotatable table 28 on which is positioned a monochromator crystal 30 to be irradiated with a beam of neutrons which passes through the neutron and inlet passageways 16 and 16a respectively. The crystal 30 reflects the beam of neutrons to form a beam of monochromatic neutrons in the well-known manner. The table 28 can be rotated about the axis of rotation 32 (see FIG. 1b to change the angle formed between one of the main faces of the crystal 30 and the beam of neutrons incident upon that face. If the crystal 30 has a reflecting face disposed at a suitable angle to the central axis of the passageways l6 and it reflects the beam in the direction of and through the outlet passageway 26.

The neutrons traveling through the inlet passageway 16a and undergoing the first-order reflection by the crystal 30 have a neutron wavelength A meeting the Braggs condition. More specifically, assuming that d represents a distance between adjacent lattice planes parallel to a crystallographic plane (hkl of a crystal involved and 0 represents the angle formed between the crystallographic plane and a direction of a beam of neutrons incident upon that plane, the above-mentioned wavelength A is expressed by the equation A 2 d sin 0 In order to continuously change a wavelength of monochromatic neutrons while a single crystallographic plane of the same crystal is used, it is necessary to continuously change the angle 0 appearing on the right-hand side of the above equation. This is inevitably accompanied by a continuous change of the angle formed between a direction in which the beam of neutrons impinges upon the crystal and the direction in which the resulting monochromatic neutrons leave the crystal, that is to say, the angle formed between the axis of the inlet passageway 16a and the axis of the outlet passageway 26.

From the viewpoint of radiation shielding, it is undesirable to increase the angular extent of the outlet passageway 26 to include the range of angles over which the neutrons will pass as the irradiated member 30 is rotated. For this reason, a plurality of movable shield elements 24a, b, c and d are inserted into the opening 22 thereby to shield the device except for an outlet through which the monochromatic neutrons traveling at an angle of 1r-2 0 to the inlet passageway 16a is extracted, that is to say, for the output passageway 26 where 0 has the same meaning as in the previous case.

The conventional device as above described was disadvantageous in that as the movable shield means are divided into several portions, in this case, four shield portions 240, b, c and d a large number of neutrons would leak externally through a clearance which might be formed between any pair of abutting movable shield elements. Upon changing an angular position of the outlet passageway 26 relative to the inlet passageway 16a, the movable shield elements 24a, b, c and d must be rearranged or replaced. This is not only troublesome but also disadvantageous in that with a collimator disposed in the outlet passageway 26 to control the divergence of the beam of monochromatic neutrons, the rearrangement of movable shield elements is accompanied by the removable of the collimator from the passageway 26 and the reinsertion of the latter into a newly formed outlet passageway. Also upon changing the angle of 1r- 2 as previously described, the adjustment of the outlet passageway 26 is accomplished through the manual rearrangement of the movable shield elements 24a, b, c and d. Accordingly the conventional device illustrated in FIGS. and b was disadvantageous in that there was not conducted any operator-less continuous measurement for a long time such as a measurement conducted with a beam of monochromatic neutrons varied in neutron wavelength.

In order to avoid these disadvantages, it has been previously proposed a neutron diffractometer as shown in FIGS. and b which will be subsequently described.

An arrangement illustrated in FIGS. 2a and b is substantially similar to that shown in FIGS. la and b except for a construction of a shield assembly. The shield assembly comprises a stationary shield block 18 having a flat surface attached to a reactor wall 12 and the opposite surface in the form of a concave cylindrical segment and a rotatable shield block 24 rotatably put in close contact with the concave surface of the stationary shield block 18. As in the previous arrangement, the stationary shield block 18 has a hole 16a aligned with a hole 16 extending through an experimental hole plug 14. The rotatable shield block 24 is provided with a central space 20, a radial outlet or extraction passageway 26 communicating with both the central space 20 and the atmosphere and a sectorial opening or an inlet passageway 34 communicating with the central space 20 and horizontally divergent toward the stationary shield block 18. The passageway 34 is always closed by the concave cylindrical surface of the stationary shield block.

The rotatable shield block 24 can be rotated about the axis of rotation 32 (see FIG. 2b to change an angle formed between a beam of neutrons incident upon a reflection face of a monochromator crystal 30 on a table 28 and that reflection face whereby the resulting monochromatic neutrons can be extracted through the outlet passageway 26. In other words, the rotation of the rotatable shield block 24 results in the adjustment of an angle formed between the inlet passageway 16a and the outlet passageway 26.

The outlet passageway 26 is fixed with respect to the rotatable shield block 24 in contrast with the arrangement illustrated in FIGS. la and b. This eliminates the necessity of moving a collimator disposed in the outlet passageway 26 upon angularly displacing the latter with respect to the inlet passageway 160. Also the sectorial passageway 34 is always closed by the stationary shield block 18 and not exposed directly to the atmosphere. Further it will be considered that the neutrons scattered from the crystal 30 or the rotatable shield block 24 is larger in number for the forward scattering than for the back scattering. Therefore it is concluded that the neutrons will less leak from the arrangement illustrated in FIGS. 2a and b than from that illustrated in FIGS. 1a and b.

However the arrangement illustrated in FIGS. 2a and b was disadvantageous in that the neutrons could much leak through longitudinal and transverse clearances which might be formed between the stationary and rotatable shield blocks 18 and 24 respectively and that a mechanism for rotating the rotatable shield block was technically different to be constructed because the latter had a weight of several tons. The invention contemplates to eliminate these disadvantages.

FIGS. 3a and b show a neutron diffractometer constructed in accordance with the principles of the invention. An arrangement illustrated comprises a stationary shield block 18 attached to a reactor wall 12 and including a hollow portion 36 of circular cross section and a rotatable shield block 24 complemental in configuration to and rotatably disposed in the hollow portion 36' with a clearance formed therebetween. As in the arrangement illustrated in FIGS. la and b, the stationary shield block 18 has a hole or a narrow inlet passageway 16a communicating with the hollow portion 36 and providing an extension of a through hole. 16 in an experimental hole plug 14 snugly fitted into the reactor wall 12 and a circumferentially wide opening or passageway 22 in the form of a slot disposed on that portion thereof remote from the reactor wall 12 and at the same level as the hole 16a. The opening 22 communicates with the hollow portion 36.

As in the arrangement illustrated in FIGS. 3a and b, the rotatable shield block 24 has a central space 20, a sectorial opening or passageway 34 opposing to the inlet passageway 16a and communicating with the central space 20, and a narrow outlet passageway 26 located at the same level as the passageway 34. The passageway 26 communicates with the central space 20 and opening into the passageway 22. A table 28 and a monochromatic crystal 30 on the table 28 are disposed in the central space 20.

With the arrangement illustrated, the rotatable shield block 24 is embedded in the stationary shield block 18 and therefore the clearance therebetween is prevent from communicating with the exterior of the device except for that portion thereof corresponding to an angular range over which the output passageway 26 for extracting a beam of monochromatic neutrons reflected from the crystal 30 is variable in angular position relative to the inlet passageway 16a. This contributes to a great increase in shielding power of the device.

While the rotatable shield block 24 is shownin FIG. 3b as 'having an upper portion in the form of a cylinder it is to be understood that the same may be of any suitable symmetry-ofrotation shape. For example, a rotatable shield block 24 shown in FIG. 4a has an upper portion in the form of a stepped cylinder while a block 24 shown in FIG. 4b has a upper and lower portion each formed into a stepped cylinder. If desired, the rotatable shield block 24 may be symmetric with respect to a plane passing through the axes of the inlet and outlet passageways 34 and 26 and have either end face in the form of a cone as shown in FIG. 4c, a concave cone as shown in FIG. 4d or a spherical segment as shown in. FIG. 4e.

The invention permits a collimator to be disposed in the extraction passage 26 and also exhibits the advantageous effect that a mechanism for rotating the rotatable shield block 24 becomes simple in construction because the shield block 24 has a light weight as compared with the arrangement illustrated in FIGS. 2a and b.

It will be apparent that if the opening 34 of the rotatable shield block 24 is close to the opening 22 of the stationary shield block 18 that there may be a fear that the shielding power will decrease to prevent this decrease in shielding power, a plurality of movable shield elements such as shown at 24a, b, c and d in FIG. 1a may be inserted into one or both of the openings 22 and 34.

FIG. 36 illustrates a plurality of rotatable shield blocks 24U, 24D vertically superposed relationship in a hollow portion 36" of the stationary shield block 18 and positioned so that inlet passageways 34U, 34D are in alignment with inlet passageways 16a U, 16a D respectively and outlet passageways 26U, 26D are in alignment with outlet passageways 22U, 22D. The stationary shield block is the inlet passageways 16U, 16D respectively in the experimental hole plug 14. Inlet passageways 34U, 34D and outlet passageways 26U, 26D terminate at the central spaces 20U, 20D within the rotatable shield blocks 24U, 24D, respectively. Within the central spaces 20U, 20D are disposed monochromatic crystals 30U, 30D respectively.

The combination of passageways 16U, 16a U, 34U and passageways 16D, 16a D, 34D define means whereby neutrons travel from the reactor core and impinge upon the monochromatic crystals 30U, 30D, respectively. The combination of outlet passageways 26U and 22U and outlet passageways 26D and 22D define means whereby neutrons reflected from the monochromatic crystals 30U, 30D, respectively leave the rotatable shield blocks 24U, 24D, respectively. Since the inlet passageways 34U, 34D and the outlet passageways 26U, 26D are wide, the paths through which the neutrons enter and leave the hollow portion 36 of the rotatable shield blocks 24U, 24D remain open even as the shield blocks 24U, 24D are rotated.

While the invention has been illustrated and described in conjunction with several preferred embodiments thereof, it is to be understood that various changes and modifications may be resorted to without departing from the spirit and scope of the invention.

We claim:

1. In a neutron diffractometer, the combination of a stationary shield block including a hollow portion, a narrow inlet passageway through which a beam of neutrons is introduced into said hollow portion, and a 'wide outlet passageway through which the beam of neutrons is extracted from said hollow portion; a rotatable shield block rotatably disposed in said hollow portion, and including a wide inlet passageway opposing to said narrow inlet passageway in said stationary shield block, a central space communicating with said wideinlet passageway, and a narrow outlet passage communicating with said central space and opposing to said wide outlet passageway; an irradiated member positioned in said central space; and a beam of neutrons passing through said narrow inlet passageway and said wide inlet passageway to irradiate said irradiated member to be reflected from the latter, the reflected beam of neutrons being extracted through said narrow outlet passageway and said wide outlet passageway.

2. In a neutron diffractometer, the combination of a stationary shield block including a hollow portion, a narrow inlet passage through which a beam of neutrons is introduced in said hollow portion, and a plurality of wide outlet passageways through which beams of neutrons are extracted from said hollow portion; a plurality of rotatable shield blocks rotatably disposed in said hollow portion, each of said rotatable shield blocks including one wide inlet passageway disposed in opposite relationship with respect to said narrow inlet passageway, a central space communicating with said wide inlet passageway, and one narrow outlet communicating with said central space and disposed in opposite relationship with respect to a different one of said wide outlet passageways; one irradiated member positioned in each of said rotatable shield block; and a beam of neutrons passing through said narrow inlet passageway and said wide inlet passageways respectively to irradiate said respective irradiated members to be reflected from the latter, each of the reflected beams of neutrons being extracted through the associated narrow and wide outlet passageways. 

1. In a neutron diffractometer, the combination of a stationary shield block including a hollow portion, a narrow inlet passageway through which a beam of neutrons is introduced into said hollow portion, and a wide outlet passageway through which the beam of neutrons is extracted from said hollow portion; a rotatable shield block rotatably disposed in said hollow portion, and including a wide inlet passageway opposing to said narrow inlet passageway in said stationary shield block, a central space communicating with said wide inlet passageway, and a narrow outlet passage communicating with said central space and opposing to said wide outlet passageway; an irradiated member positioned in said central space; and a beam of neutrons passing through said narrow inlet passageway and said wide inlet passageway to irradiate said irradiated member to be reflected from the latter, the reflected beam of neutrons being extracted through said narrow outlet passageway and said wide outlet passageway.
 2. In a neutron diffractometer, the combination of a stationary shield block including a hollow portion, a narrow inlet passage through which a beam of neutrons is introduced in said hollow portion, and a plurality of wide outlet passageways through which beams of neutrons are extracted from said hollow portion; a plurality of rotatable shield blocks rotatably disposed in said hollow portion, each of said rotatable shield blocks including one wide inlet passageway disposed in opposite relationship with respect to said narrow inlet passageway, a central space communicating with said wide inlet passageway, and one narrow outlet communicating with said central space and disposed in opposite relationship with respect to a different one of said wide outlet passageways; one irradiated member positioned in each of said rotatable shield block; and a beam of neutrons passing through said narrow inlet passageway and said wide inlet passageways respectively to irradiate said respective irradiated members to be reflected from the latter, each of the reflected beams of neutrons being extracted through the associated narrow and wide outlet passageways. 